More than a third of stage I-III breast cancer patients have bone marrow micrometastases at the time of diagnosis providing a source of recurrence. Most recurrences occur in post-menopausal women. Mechanisms of dormancy and recurrence are not well understood, but data suggest a dependence on a close association with bone marrow stroma. We hypothesize that stromal cells undergo senescence due to aging and/or post-menopausal estrogen deprivation and begin to secrete inflammatory cytokines that can stimulate dormant cancer cells to re-awaken. The broad, long-term goals of our investigations are to define mechanisms that govern the establishment of the dormant state in breast cancer cells in the bone marrow and to determine factors and mechanisms responsible for their re-awakening and recurrence of disease. We propose to determine if bone marrow stroma can undergo senescence when deprived of estrogen or treated with cytotoxins in vitro and in vivo in a murine model. Our specific aims are: 1. to determine if in vitro estrogen deprivation can induce a senescent phenotype in bone marrow stromal cultures incapable of supporting breast cancer dormancy in an in vitro model and 2. to determine if in vivo estrogen deprivation induces a senescent phenotype in bone marrow stroma rendering it incapable of supporting breast cancer dormancy in vitro and in vivo. We will establish and characterize the phenotype of secretory senescence by subjecting stromal monolayers to oxidative and hypoxic stress and estrogen deprivation and measure the expression and activation of TGF[unreadable], Cox-2, IL-6, IL-8 and SA-[unreadable]Gal, known markers associated with senescence. We will determine if estrogen deprivation in vitro and in vivo and cytotoxicity in vitro can induce senescence measured by these molecular markers and by the loss of support of breast cancer dormancy in an in vitro clonogenic co-culture model and in a left ventricle injection bone marrow metastasis model. Experiments will also determine whether estrogen-deprivation renders stroma more susceptible to chemical injury and whether administration of Cox-2 inhibitors or estrogen can reverse these effects. These studies will establish a way of thinking about dormancy as a function of the senescent microenvironment and seek to reverse estrogen-deprivation-induced inflammation to maintain it. PUBLIC HEALTH RELEVANCE: The proposed study will investigate the induction of senescence in mouse bone marrow stroma by estrogen deprivation in vitro and in vivo as manifested by the secretion of inflammatory cytokines and loss of the capacity to support dormancy of breast cancer cells in an in vitro model and the loss of the capacity to support the dormancy of xenografted human breast cancer cells in the bone marrow microenvironment. Experiments will determine if treatment with estrogen or anti-inflammatory agents can restore the capacity of senescent stroma to support dormancy.